The I-I bond distance in I2 (gas phase) is 267pm, in the [I3]+ ion is 268pm, and in [I3]- is 290pm.
a) draw the lewis structures for these species. Do these representations account for the variation in bond distance?
b) Use MO diagrams for each structure to describe the bonding and deduce the I-I bond order in each species. Are your results consistent with the structural data?